Dietary niche overlap in a nearshore elasmobranch mesopredator community

Abstract

Understanding mesopredator trophic interactions is crucial to understanding the dynamics of food webs because mesopredators provide the link between apex predators and lower trophic levels. Using stable isotopic analysis and stomach content analysis, we examined dietary niche overlap within a diverse elasmobranch mesopredator community in Shark Bay, Western Aus- tralia. Isotopic values (δ 13 C and δ 15 N) were consistent with most species being highly dependent on a seagrass-based food web. Differences were observed between the mean isotopic values of species, but there was a great deal of overlap in the isotopic niche space used by the community when exam- ined at the level of individuals. Stomach contents also suggest dietary overlap among the diets of Himantura spp. and Glaucostegus typus, which contained many of the same prey species, primarily crustaceans typically associated with seagrass habitats, although in different proportions. Diet data also suggest that, despite having isotopic values similar to other species, Pastinachus atrus appears to feed on sandflat-associated species. In this community, variation within the groups examined, possi- bly due to individual specialization, appears to result in high resource overlap and may be a key com- ponent allowing for high diversity in this system and is perhaps crucial to understanding the role of mesopredators in community trophic dynamics.